![]() ![]() ![]() ![]() Put a tiny drain and narrow drain pipe on and you are going to wait a long time to drain the tub. The water is going to drain much faster if you have a very wide drain and pipe to drain the water. It's like draining a bathtub full of water. You could have a big-ass battery with lots and lots of material, but if the electrode surface is still tiny, then you will NOT pass any high currents at all. Yes yes fine, there are other factors too but the main one is surface area of the electrode. It is the surface area of a battery that ultimately limits the maximum current possible from the battery. It is important to remember that ALL the electrons entering and leaving a battery HAVE to go to and from the two reactants via the electrode SURFACE. You DO pay the cost in size and weight though, now you are at 139 grams for the weight which is about 6× more than the AA. If you go with a D alkaline, you'll get 16000 mAh. you can either add more AA batteries OR use a higher capacity battery like a C or D cell. Now say you want more current (capacity) in your set up. A AA battery weighs about 23 g total so the other 14 g is the materials holding it all together, plastic, stainless steel, more plastic + the electrolyte, water, and a little excess Zn and MnO 2 for good measure. So only about 9 grams of actual material is being converted to products in a AA battery. ![]() Now we can convert that into the amount of Zn and MnO 2 which is 2.4 g of Zn and 6.5 g of MnO 2. If we use our alkaline cell redox reaction (see last section) we know that 2 moles of electrons are passed for the overall reaction. a 2000 mAh AA battery means you are running 25 mA for 80 hr. Yes, from your reactants in the redox reaction. Size MattersĪlso remember that those electrons zipping around your circuit have to come from somewhere. Alkaline batteries are NOT so great in high-drain devices like digital cameras because you'll only get about 1/2 or less of the advertised capacity. So when you are comparing bang for your buck with batteries, you need to know the current drain of your device and then check the capacity of the battery under those conditions. Bump that drain-rate up to 500 mA and you'll only get 1300 mAh which is less than half the capacity at the smaller 25 mA rate. Yes, the drain-rate is important and it affects the actual capacity of the battery. Which is true only IF you are only pulling 25 mA or less during the operation. Here is a typical AA alkaline battery capacity written out for you: So multiply the current in milliamps by the time in hours and you'll get mAhr which IS the go-to standand for battery capacities in this size of application (small electronics/toys/devices). And, most "little" batteries that we use (the AAA, AA, C, D) only pass a wee bit of current usually in their devices which means you'll get better numbers if you use milliamp-hours or mA We say amp-hours which is just straight up current units times time units in hours, an amp-hr or Ahr. The real world only uses the top part of that equation (the numerator) and calculates coulombs of charge passed which is justĪnd we don't even say coulombs either (sigh). We learned earlier that you can calculate the total number of moles of electrons being pumped around a circuit by using current times time divided by the faraday.Īnd as much as I'd love a world that counted in moles, it just doesn't work that way. For a redox reaction that is providing electric current, we can get at those moles with some math. We chemistry will usually think in terms of moles of this or that when talking amounts of stuff. ![]()
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